ABSTRACT Challenging auditory environments, e.g., classrooms, school cafeterias, playgrounds, comprise a significant portion of a child's day. Binaural hearing facilitates speech understanding in these environments where sounds and speech overlap, originate from multiple sources, and vary in level. Binaural auditory cues are necessary to understand speech and locate talkers in acoustically complex settings; however, binaural cues are degraded or eliminated for children with asymmetric hearing loss (AHL, defined as severe to profound hearing loss in one ear and mild to moderate hearing loss in the other) or single-sided deafness (SSD, defined as severe to profound hearing loss in one ear and normal or near normal hearing in the other). A cochlear implant (CI) is the only treatment that can provide hearing to an ear with severe to profound hearing loss and thus, the only opportunity for binaural hearing. Yet, children with AHL/SSD are not routinely implanted in the poor ear because the better ear benefits from amplification (AHL) or has normal hearing (SSD). Recently, the FDA granted approval to implant a subset of children with AHL/SSD; however, the approval is restrictive as it is limited to children with profound hearing loss and ? 5% word recognition in the poor ear. Furthermore, there is no consensus regarding CI candidacy criteria, assessment tools, or performance outcomes over time for these children. A longitudinal, prospective, multi-center clinical trial employing an FDA-approved protocol is critically needed to address deficits of children with AHL/SSD and explore treatment with a CI. Our proposed sequential, two-phase study allows for comparison of performance growth pre-implant with current hearing aid (HA) technology versus post-implant with a CI. Aim 1 obtains preliminary efficacy data in children with AHL/SSD who receive a CI in the poor ear. Post-implant performance with a CI alone is compared with pre-implant performance with a HA. Measures include word recognition and sound field detection levels. Aim 2 evaluates the effectiveness of bimodal hearing defined as a CI in the poor ear and a HA in the better ear (AHL) or a CI in the poor ear and normal hearing in the better ear (SSD). Bimodal efficacy is measured by comparing: 1) post-implant bimodal scores to pre-implant best-aided scores, 2) post-implant, bimodal scores to better ear alone scores, and 3) change in bimodal performance across time (CI phase) to change in best-aided performance over time (HA phase). Measures encompass speech recognition in noise and at soft levels, localization, and Quality of Life metrics. Essential information related to patient selection, test measures, and methodology will inform clinical management of these underserved populations. In summary, study outcomes will provide crucial evidence-based data regarding the development of binaural hearing abilities in children with AHL or SSD who receive a CI in the poor hearing ear, which is integral to the establishment of standardized treatment.